The most common pollutant of the lubricating oil used in diesel engines is soot. Diesel engine lubricating oils normally contain a dispersant for maintaining the soot particles in suspension. It is important, for economical operation of such an engine, to be able to determine when the dispersant has been substantially used up. In this manner, the lubricating oil can be changed at the most economically advantageous time. Changing the oil substantially prior to the full utilization of the dispersant is wasteful but going beyond such point might cause damage to the engine. Accordingly, it would be helpful to have a simple method of testing the soot content of a small sample of lubricating oil.
It would also be important that any such test be run quickly and that it be accurate and inexpensive. Such a test would be particularly useful, for example, for operators of long haul trucks.
It might appear obvious to make a simple optical determination of soot particles in oil by placing an oil sample in a conventional cell bounded by parallel glass plates and shining a light therethrough to a detector. However, that is not the case. For example, when new or only slightly used diesel oil is examined in such a cell, the oil is frequently too clear to permit an accurate measurement. On the other hand, oil which has been used for some time is often too opaque to determine the soot concentration. As a result, such a cell is usable only to measure a limited range of concentrations. This problem may be overcome by using several different cells or by carefully diluting the oil samples.
Accordingly, it is a primary object of the present invention to provide an improved optical cell for determining soot concentration in lubricating oil other objects are to provide such a cell which is simple in construction, which is simple to use, which is usable through a wide range of soot concentrations, which does not require calibration against ano cell or instrument, and which produces accurate and reproducible results. The manner in which these objects are achieved will be apparent from the following description and appended claims.